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Unregistered
March 30th, 2005, 11:44 AM
HELLO,

LET ME GET THIS STRAIGHT, IF I HAVE A POWER SUPPLY THAT OUTPUTS 24VDC AND 5A TO OUTPUT MODULES THAT DRAIN 0.5A. is this ok?

i guess what im trying to say is, is it safe to have an extra current supply than needed for the PLC modules,

i have a simatic s7-300 PLC with a sitop 5A PSU

thanks

Rod
March 30th, 2005, 12:06 PM
The devices or loads will only consume what their rated loads are.

A 100 milliamp load is .1 Amp
1 Amp = 1 Amp

So you take all the loads and add them up:
1 + .5 + 2 + 2.5 = 6 amps, you need a 6 amp power supply PLUS maybe %50 more to be safe, or 9 amps. If the nearest supply to that is 10 amps then that should be correct.

Rod (The CNC dude)

EDIT!! PLUS the loads of the output/input devices!!! Me so DUM

DonsDaMan
March 30th, 2005, 12:34 PM
I had a power supply once that needed a minimum load in order to operate. It was rated for about 10 amps, but below 2 amps, it would shut down.

I don't know how it worked, since I trashed it and got a better supply that could handle lower currents.

Unregistered
March 30th, 2005, 12:40 PM
ic, but interms of IO modules only, is it the ouputs that are the only current consuming modules? so basicly having a PSU(5A) of higher current supply than my load will be ok ie safe for load of about total 1.5A

MrQ
March 30th, 2005, 12:47 PM
Of course the CPU and in/out modules need some current to. This you can find out from a datasheet for the modules.

The large current however is what the loads need for example a magnet on a valve needs 1-2 Amps. If you have 10 valves multiply by 10.

However if you are SURE that Your PLC programs only activates max 5 valves at a time you can dimension for this.

Rod
March 30th, 2005, 12:55 PM
I best step back from this.

DonsDaMan (switcher type suppies do require some minimum load current to operate) You can fool them with a shorting resistor - I'm not getting into this further as I don't know this PSU.

If I could put my meter on it and read the data sheets, it would be easy.

Sorry

Rod (The CNC dude)

danw
March 30th, 2005, 06:57 PM
Yes, you are safe to use a 5 amp supply for a load of 0.5 amps. The power supply only supplies the amperage needed for the load(s).

The amp rating of the power supply must be greater than the total load.
5 amps (rating) > 0.5 amps (load), so you're OK.

You could theoretically run up to 10 of the 0.5 amp loads (10 x 0.5 = 5), but many people won't run a power supply at its rated output.

Terry Woods
March 30th, 2005, 07:29 PM
"...I HAVE A POWER SUPPLY THAT OUTPUTS 24VDC AND 5A..."

This simply means that your power supply is capable of delivering UP TO 5A while maintaining the 24V output voltage level. If the load exceeds 5A then your voltage will begin to dip.

A Power Supply is a Power Provider... as in I x E = Power (Watts). In this case, 5A x 24V = 120 Watts.

This means that the Power Supply will maintain 24V as long as the current load is 5A or less.

If you add too much loading you might find, for example, that the supply is delivering (or trying to deliver) 6A. Since the Power (Watts) remains the same, your voltage will be 120 W / 6A = 20V

Some Power Supplies will allow this higher amperage/lower voltage situation for just a short time before it either blows an on-board fuse or trips an on-board circuit breaker.

Some Power Supplies have "loading-logic" built-in that will shut the supply down until the excessive load is removed. Some of this kind of supply has an Auto-Reset... other have to be manually reset.